1. Field of the Invention
The invention relates to a method for biological purification of municipal or like wastewater by means of activated sludge, wherein the wastewater is introduced first into an aerated activated sludge tank (B tank) and then by turns into one of several sedimentation and recirculation tanks (SU tanks), which are permanently linked with said B tank and in which, several times a day an operating cycle proceeds comprising a stirring phase (R phase), a pre-settling phase (V phase) and a discharge phase (A phase), wherein in turns in the R phase the activated sludge is remixed with the water, in the V phase the activated sludge settles down and in the A phase clear water is drawn off and wherein the cycles in the SU tanks are phase-displaced to each other and the A phases are adjacent, only in the A phases the SU tanks are flown through, an approximately constant water level is present, thus causing an outflow from the purification plant corresponding to its inflow (throughflow principle).
2. Description of the Prior Art
From the European patent application EP 968 965 a method for biological wastewater purification by means of activated sludge is known where the wastewater is introduced first into an aerated activation tank and then into a settling tank, in which a separation of activated sludge and clear water occurs and after separation activated sludge is returned into the activation tank and clear water is discharged. Several times a day an operating cycle is performed, comprising a stirring phase, a secondary settling phase and a discharge phase, wherein in the stirring phase the activated sludge is remixed with the water, in the secondary settling phase the activated sludge settles down and in the discharge phase clear water is drawn off. According to the above method of prior art, the purification is done in a biological two-tank systemxe2x80x94the activation and the sedimentation tanks with continuous inflow and intermittent outflow. In the intervals without outflow the water level rises due to the inflow (damming-up principle). The patent claim of this method consists in that after the pre-settling phase and before the stirring phase settled activated sludge is returned into the activation tank of the xe2x80x9ctwo-tank system with damming-up operationxe2x80x9d. That this method relates to a damming-up operation can be seen from the description of the document (pages 14 and 15) as well, stating: xe2x80x9cthat water is introduced permanently to the first region and from there it spills into the second region. A discharge of purified drain water is performed here only during the third step of the method. During the other steps the drain water accumulates in both regions orxe2x80x94in the case of presence of an anaerobic pre-treatmentxe2x80x94also in this region.xe2x80x9d Also in claim 1 it is clearly evident that it concerns xe2x80x9ctwo-tank systems with damming-up operationxe2x80x9d which xe2x80x9care connected in parallel and operated time-delayed.xe2x80x9d This method of prior art is very suitable for small purification plants. For middle or large-scale purification plants, however, it is far better to use the throughflow principle. Then the outflow from the purification plant corresponds to the inflow.
A similar method is known from the WO 97/08104, where at the beginning of each cycle the same sludge concentration is adjusted in the activation and sedimentation tanks, the reintroduction of the non-settled activated sludge occurring during the stirring phase. A reintroduction of settled and well-thickened activated sludge before the stirring phase is not provided for.
Furthermore, a similar method is known from the European patent EP 0 670 817 B1 of Dec. 29, 1999, where the wastewater is treated in two cells, wherein the wastewater is aerated an mixed in the treatment and discharge cell and wherein the reintroduction of sludge from the treatment and discharge cell into the first treatment cell occurs during the mixing period (B and R phase). Here it is essential that cell aeration and mixing is done in the treatment and discharge and no settled and thickened activated sludge is let to reintroduction, which is why a longer time is needed for the reintroduction and a smaller content of dry substance in the first treatment cell is achieved, thus a loss in time for the other phases comes about (compare claim 1 of the document).
A similar method is known from the European application EP 1 110 916 of Jan. 17, 2000. In a purification plant operating according to the throughflow principle and exploiting the one-basin technology, settled and thickened activated sludge is returned after the V phases and before the R phases into the first treatment tank. The reintroduction of sludge is done in a relatively short time, which makes a large return amount necessary.
The EP 0 399 013 relates to a facility for wastewater processing, in which buffering of larger amounts of wastewater (wastewater impacts) is possible in a simple way; this is achieved in that the closure means of the outlet of the activation tank comprising a moveable closure body made from an elastically deformable foil. The fluid from the activation tank is transported into the secondary clearing tank by means of a air-jet lift. On the bottom of both tanks a closeable opening is provided which serves for transporting settled sludge from the secondary clearing tank into the activation tank and which is opened only for a short time. Thus, the connection of the two tanks is hydraulically interrupted and causes different water level positions in the two tanks. The method underlying this facility is an activation method operating according to the damming-up principle with intermittent, short-time reintroduction of sludge from the post-clearing tank into the activation tank, as opposed to the throughflow method as cited in the beginning.
The invention is based on the problem to improve the above-described methods for biological wastewater purification in a manner that allows application in middle and large-scale purification plants by using the throughflow principle and, at the same time, achieving a higher sludge concentration in the activation tank at a shorter reintroduction time through the reintroduction of settled and well-thickened activated sludge. This problem is solved by the features of claim 1, according to which after the V phases and before the R phases, settled and thickened activated sludge is reintroduced into the B tank from the SU tanks (S phase).